1. Field of the Invention
The present invention relates to a wavelength-multiplexing optical transmission system, for use in an optical communication system, for transmitting a plurality of wavelength-multiplexed optical signals.
2. Description of the Prior Art
Recently, an optical fiber communication system has been remarkably developed. This system has been thus applied to and put to practical use for a public communication, CATV, a computer network or the like. The optical fiber communication system includes a wavelength-multiplexing optical transmission system, in which multi-channel signals are transmitted through one optical fiber cable. An optical transmitter wavelength-multiplexes respective channel signals by using an optical signal having a different wavelength so as to transmit the signal to an optical receiver through one optical fiber cable. The optical receiver selectively demultiplexes and demodulates the optical signal having a desired wavelength by a wavelength selector. Furthermore, a high-speed Local Area Network (LAN) system having a switching function and a smaller cost is constructed by using the wavelength selector which taps the optical signal alone of a predetermined wavelength but lets the other optical signals pass therethrough. This is disclosed, for example, in the prior art document 1, Masahiro EDA et al., "A Hybrid Optical Local Area Network Using Photonic Wavelength-Division Multiplexing Technique", Technical report of IEICE (Institute of Electronics, Information and Communication Engineers), OQE-91-126 and OCS-91-61, pp. 61-68, 1992.
FIG. 10 is a block diagram showing a constitution of a wavelength selector for use in a prior art wavelength-multiplexing optical transmission system.
Referring to FIG. 10, the prior art wavelength selector comprises an optical circulator 80 and a fiber grating 82 including a diffraction grating formed in an optical fiber cable 81. The optical circulator 80 is a circulator for circulating the optical signal in the clockwise direction. The optical circulator 80 includes a first port, a second port, and a third port which are arranged in the clockwise direction. The optical signal is inputted to the first port thereof, and the optical fiber cable 81 is connected to the second port thereof. The optical signal having a predetermined wavelength is reflected by the fiber grating 82 of the above-described optical fiber cable 81, and then outputted from the third port of the optical circulator 80. On the other hand, the optical fiber cable 81 transmits optical signals other than the reflected optical signal having the predetermined wavelength as described above. These optical signals are then outputted from the other end of the optical fiber cable 81. As well known, the fiber grating 82 comprises a periodical refractive index distribution, which is formed in a core of the optical fiber cable 81 by ultraviolet rays. A formation of the fiber grating 82 is disclosed, for example, in the prior art document 2, Akira INOUE et al., "Fabrication and Application of Fiber Bragg Grating", Technical report of IEICE (Institute of Electronics, Information and Communication Engineers), OPE-94-5, pp. 25-30, May 1994. The prior art reference 2 discloses that a tension force is applied to the fiber grating or the fiber grating is heated, and then this leads to a changing of a pitch of the fiber grating and thus a reflected or transmitted wavelength can be changed.
FIG. 11A is a wavelength spectrum diagram showing wavelength reflection characteristics of the wavelength selector of FIG. 10, and FIG. 11B is a wavelength spectrum diagram showing wavelength transmission characteristics of the wavelength selector of FIG. 10. As apparent from FIG. 11, the wavelength selector has the characteristics for selecting the wavelength of a steeply narrow band. Now, its effective index is indicated by n and its period of the refractive index distribution is indicated by P. In this case, the central wavelength .lambda.B of the reflected wavelength is given by the following equation (1). EQU .lambda.B=2.times.n.times.P (1).
Moreover, a stress is applied to the fiber grating 82 or the fiber grating 82 is heated, and then, this leads to a change of the effective index n, and thus the reflected wavelength can be changed.
FIGS. 12A and 12B are a wavelength spectrum diagrams showing a wavelength selection of the optical signal having one wavelength in the prior art wavelength-multiplexing optical transmission system which selects an wavelength by using the wavelength selector of FIG. 10 or a conventional periodic one such as a Fabry-Perot etalon, wherein FIG. 12A shows wavelength selection characteristics of the wavelength selector of FIG. 10, and FIG. 12B shows a wavelength-multiplexed optical signal which is allowed to pass through the wavelength selector of FIG. 10.
As apparent from FIGS. 12A and 12B, a wavelength interval .DELTA..lambda. is set to be sufficiently wider than a wavelength interval .delta..lambda., where .DELTA..lambda. denotes the wavelength interval between two adjacent reflected wavelengths in the wavelength selection characteristics of the wavelength selector, and .delta..lambda. denotes the wavelength interval between two adjacent optical signals of a wavelength-multiplexed optical signal 15. That is, the prior art wavelength-multiplexing optical transmission system is constituted in such a manner that the reflected wavelength of the wavelength selector is changed so as to thereby reflect and filter one optical signal alone of a plurality of optical signals 15a to 15f included in the wavelength-multiplexed optical signal 15.
However, the temperature dependence of the wavelength selection characteristics of the fiber grating 82 is about 0.01 nm/deg. The dependency of the tension force thereof is about 0.013 nm/g. Accordingly, for example, when a variable wavelength range of 5 nm is obtained, the temperature and the tension force must be changed, respectively, by about 500 degrees and by 400 g. The optical fiber cable 81 itself maybe therefore damaged. Therefore, the actually variable wavelength range becomes narrower. Thus, the number of optical signals, which can be wavelength-multiplexed, is limited. Consequently, a transmission capacity for an optical transmission apparatus cannot be increased. That is, this results in the wavelength-multiplexing optical transmission system as shown in FIGS. 12A and 12B.